Protected isotope heat source

ABSTRACT

A radioactive isotope capsule is disposed in a container (heat shield) which will have a single stable trim attitude when reentering the earth&#39;&#39;s atmosphere and while falling to earth. The center of gravity of the heat source is located forward of the midpoint between the front face and the rear face of the container. The capsule is insulated from the front face of the container but not from the rear surface of the container.

Burns et al.

1 1 PROTECTED ISOTOPE HEAT SOURCE [75] Inventors: Raymond K. Burns, Lakewood;

Lloyd 1. Shure, Willowick, both of Ohio; Elliott D. Katzen, Palo Alto, Calif.

[73] Assignee: The United States of America as represented by the United States Aeronautics and Space Administration, Washington, DC.

[22] Filed: May 26, 1971 [21] App]. No.: 146,939

[52] U.S. Cl. 250/496; 244/1 SS; 250/493 [51] Int. Cl G211] 5/00; G21f 5/02 [58] Field of Search 250/106 S, 106 VC, 108 R, 250/108 WS, 496, 493; 244/] SS; 136/202; 176/78 [56] References Cited UNITED STATES PATENTS 2,910,593 10/1959 Laing et a1 250/106 S [4 1 Aug. 12, 1975 3,253,152 5/1966 Lahr 250/106 S 3,451,641 6/1969 Leventhal 136/202 3,488,502 1/1970 Dukes 4 250/106 S 3,569,714 3/1971 Anderson 250/106 S 3,596,853 8/1971 Anderson 244/1 SS 3,621,261 11/1971 Princiotta 250/106 5 Primary Examiner-Harvey E. Behrend Attorney, Agent, or FirmN. T. Musial; J, A, Mackin [57] ABSTRACT 10 Claims, 2 Drawing Figures RBSE-LESEBO PATENTED AUG I 2|975 N .QE Ti Ti on I llllllllllll A V///U. 0N 5 mm g mm PROTECTED ISOTOPE HEAT SOURCE ORIGIN IN THE INVENTION The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government of the United States for governmental purposes without payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The invention relates to radioactive isotope capsules and is directed more particularly to a protective arrangement which prevents an isotope heat source from being destroyed by excess heat while falling through the earth's atmosphere.

Radioactive isotopes having a protective sheath are used in space vehicles as heat sources. These heat sources provide energy for the purpose of, for example, heating an inert gas to provide the driving force for turbines and other turbine driven equipment such as generators. In cases where capsules may return to Earth, reentry heat shields are provided to protect the capsule from the high temperatures encountered as it falls through the Earths atmosphere.

Up to the present time; protection has been provided for radio-active isotope capsules by disposing them in containers made of a high heat conductivity material which is also lightweight. as for example, graphite. In these prior art protective devices. the isotope capsule is surrounded by insulation within the container to limit aerodynamic heating of the container from being trans ferred to the capsule when it falls through the Earth's atmosphere. The necessity of completely surrounding the capsule with this insulating layer in prior art devices resulted from the possibility of their stabilizing in numerous and unpredictable orientations during atmsophereic reentry. This aerodynamic heating by itself could be great enough to damage or destroy the protective sheath on the isotope capsule if insulation were not prtl'vltled.

Because of this insulation which completely surrounds the capsule. the thermal energy of the capsule must be transferred through it during steady state oper 'ution with the power conversion device in space. This can result In a high and deleterious operating temperature for the capsule.

SUMMARY OF THE INVENTION It is an object of the invention to provide for a radioactive isotope capsule a reentry container which will prevent excessive heat from being transferred to the capsule during a fall through the Earth's atmosphere and will at the same time provide for a reduced temperature gradient and rapid heat transmission from the capsule to a utili/ation point during operation than has been possible in the past.

It is another object ofthe invention to provide a reentry isotope heat source which will enter and fall through the Earth's atmosphere such that if its orientation stabili/es it will be in a single. predetermined attitudc.

Still another object of the invention is to provide a reentry container for an isotope capsule in which heat transfer from the capsule to the container and vice- \crsn LII t; IIUI'IUIIIIUTIII fill It is yet another object of the invention to provide a reentry isotope heat source having predetermined aerodynamic characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a transverse cross-sectional view of a typical embodiment of the invention.

FIG. 2 is a longitudinal cross-sectional view of the typical embodiment.

DESCRIPTION OF A TYPICAL EMBODIMENT OF THE INVENTION Referring now to FIG. 1 there is shown a cylindrical radioactive isotope capsule I0 disposed in a reentry container II. The isotope capsule 10 is protected by an exterior sheath (not shown) as for example, platinum. The container 11 has a flat forward face 12, flat side surfaces 13, 14 and a flat rear face 15. Curved surfaces l6 and I7 merge the side surfaces l3, 14 into the flat rear face 15. The purpose of the curved surfaces 16 and I7 is to provide aerodynamic characteristics for the container 11 such that the flat forward face 12 will face forward if a stable orientation occurs as the heat source falls through the atmosphere.

The radii of curved surfaces I6 and 17 may emanate from respective points on lines 18, 19 representing imaginary planes which extend from the center 20 of the container 11 as defined by the vertical dashed line 21 and the horizontal dashed line 22, however, this is not essential. The physical center of the capsule I0 is defined by the vertical line 21 and the horizontal line 23. The physical center of the capsule 10 is also the center of gravity of the container-capsule structure because the capsule is made of extremely heavy metals while the container is made of a light material such as a graphitic ablative material.

As shown in FIG. I, the isotope capsule 10 is adjacent to the cavity surface in the container ll over a l are to the rear of line 23 as shown at 24. Thus.

the radius of the cavity to the rear of line 23 is substantially the same as the radius of the capsule and the capsule is consequently in direct contact with the cavity wall as shown as 24. To the front of line 23. the radius of the cavity is substantially greater than the radius of the capsule to allow for a layer of thermal insulating material 25 to be disposed between the capsule and the cavity surface 26. Thus the capsule is substantially smaller in diameter than the cavity.

Referring now to FIG. 2 it will be seen that the capsule 10 is an elongated cylinder having dome-shaped end portions 27 and 28. The ends of the container 11 are closed by end plugs 29 and 30. As shown. the insulating layer 25 extends into recessed portions 3] and 32 of the plugs 29 and 30. respectively. The insulating layer is typically made of pyrolytic graphite while the ends plugs 29 and 30. like the container I]. are typically formed from a graphitic ablating material.

In operation in a space vehicle. heat from the capsule I0 is transferred directly to the cavity surface 24 and then to a utilimtion point from the rear surface [5 of the container II as represented by the arrows 33. This arrangement results in a direct heat transfer path. which minimizes the operating temperature of the cap sule. On the other hand. if the container while falling through the Earths atmosphere. because of its center of gravity and shape stabili/es, it will be with the face l2 forward. Accordingly. the maximum amount of heat is generated in the front portion of the container 1 l as indicated by arrows 34 when the container is reentcring the Earths atmosphere in this attitude. However, the insulating layer 25 prevents a high rate of heat exchange between the capsule and the front portion of the container so that the capsule is not damaged because of excessive heat.

ALTERNATE EMBODIMENTS OF THE INVENTION In an alternate embodiment of the invention, the cap sule may be a sphere, in which case the container 11 could be reduced in length. As another alternative, two or more spherical capsules could be disposed in the cavity of the container 11.

From the foregoing, it will be seen that the invention provides a radioactive isotope heat source in which heat is advantageously transferred effectively to the rear portion of a container of an isotope capsule during operation and in which a minimum amount of heat is transferred from the front portion of the container to the capsule during reentry through the Earth's atmosphere in a stabilized attitude.

It will be understood that those skilled in the art may change or modify the isotope heat source described above without departing from the spirit and scope of the invention as set forth in the claims appended hereto.

What is claimed is:

1. An isotope heat source for atmospheric reentry comprising:

a container having a cavity therein and having a flat front face, flat side surfaces and a rear face which merges with said flat side surfaces along respective curved surfaces which extend from one end of the container to the other;

an isotope capsule disposed in said cavity of said container, said capsule being substantially smaller than said cavity, and being in direct contact over a substantial portion of its surface with the cavity wall adjacent said rear face of said container; and

a heat insulating layer disposed between said capsule and the well of said cavity adjacent said flat front face.

2. The isotope heat source of claim 1 wherein said container is elongated and a cavity extends longitudinally through said container from end to end, a plug being disposed in each end of said cavity.

3. The isotope heat source of claim 2 wherein a recess is provided in each end plug and said insulating layer extends into said recesses between the ends of the capsule and said end plugs.

4. The isotope heat source of claim 2 wherein said cavity is cylindrical.

5. The isotope heat source of claim 4 wherein the portion of said cavity to the rear of a plane parallel to said front face has a radius of predetermined length and the portion to the front of said plane has a substantially larger radius.

6. The isotope heat source of claim 5 wherein said isotope capsule has a radius equal to that of said portion of said cavity to rear of said plane, said capsule being in direct contact with the wall of said cavity having the smaller radius over l of the surface of said capsule.

7. The isotope heat source of claim 6 wherein said heat insulating layer is disposed between said capsule and the wall of said cavity having the larger radius.

8. the isotope heat source of claim I wherein said container is an ablative material.

9. The isotope heat source of claim 1 wherein said insulating layer is pyrolytic graphite.

10. The isotope heat source of claim 1 wherein the radius of curvature of said curved surfaces is equal to the distance from the physical center of the container to said side surfaces and to said rear face. 

1. An isotope heat source for atmospheric reentry comprising: a container having a cavity therein and having a flat front face, flat side surfaces and a rear face which merges with said flat side surfaces along respective curved surfaces which extend from one end of the container to the other; an isotope capsule disposed in said cavity of said container, said capsule being substantially smaller than said cavity, and being in direct contact over a substantial portion of its surface with the cavity wall adjacent said rear face of said container; and a heat insulating layer disposed between said capsule and the well of said cavity adjacent said flat front face.
 2. The isotope heat source of claim 1 wherein said container is elongated and a cavity extends longitudinally through said container from end to end, a plug being disposed in each end of said cavity.
 3. The isotope heat source of claim 2 wherein a recess is provided in each end plug and said insulating layer extends into said recesses between the ends of the capsule and said end plugs.
 4. The isotope heat source of claim 2 wherein said cavity is cylindrical.
 5. The isotope heat source of claim 4 wherein the portion of said cavity to the rear of a plane parallel to said front face has a radius of predetermined length and the portion to the front of said plane has a substantially larger radius.
 6. The isotope heat source of claim 5 wherein said isotope capsule has a radius equal to that of said portion of said cavity to rear of said plane, said capsule being in direct contact with the wall of said cavity having the smaller radius over 180* of the surface of said capsule.
 7. The isotope heat source of claim 6 wherein said heat insulating layer is disposed between said capsule and the wall of said cavity having the larger radius.
 8. the isotope heat source of claim 1 wherein said container is an ablative material.
 9. The isotope heat source of claim 1 wherein said insulating layer is pyrolytic graphite.
 10. The isotope heat source of claim 1 wherein the radius of curvature of said curved surfaces is equal to the distance from the physical center of the container to said side surfaces and to said rear face. 